The present invention relates generally to separating, collecting and processing materials, and more specifically to treatment of electric arc furnace dust.
One way to produce steel is to refine iron in an electric arc furnace (EAF), in which heat is supplied by arcs struck through the molten metal between carbon electrodes. Melting and refining proceed simultaneously after the solid charge is submerged below a layer of molten metal. This procedure is used primarily for refining steel scrap and direct reduced iron. Very high temperatures are generated in the arc plasma and volatile species are effectively removed from the metal. Dust, called EAF dust, is generated during the process, and collected by a baghouse.
In a recent market research survey of 76 EAF shops in the U.S., a total of 1,069,457 tons of EAF dust were generated by the plants surveyed in 1999. For all EAF shops in the US and Canada, it was estimated 1.2 million tons of EAF dust were generated in 1999. Overall, 82% of the EAF shops responded that between 25 to 44 pounds of EAF dust per ton of steel is produced. The amount of EAF dust generated is expected to increase each year as increasing amounts of steel is produced from galvanized steel scrap in EAF shops. The increasing use of galvanized steel scrap has resulted in higher levels of zinc in the EAF dust.
EAF dust has a complex mineralogy. Prior research has identified, by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, the major components of EAF dust are predominately iron oxide, zinc oxide and zinc ferrite with well defined peaks of sodium chloride and potassium chloride present. Because calcium oxide is added to desulfurize and remove silica from the steel in the EAF, calcium compounds are predominant in various amounts in the EAF dust. Chloride can exist in a calcium chloride phase with magnesium, iron, zinc, lead and cadmium.
Since EAF dust contains hazardous materials, such as lead and cadmium, it is a costly and time consuming problem for steel manufacturers. Although many processes have been reported, the most common ways to deal with EAF dust is shipping it to an offsite processor for landfill or recycling. In addition to the environmental reasons, it would be economically and environmentally desirable to find an alternative to disposal because the components of EAF dust are valuable when separated and recovered. One of the constituents of EAF dust that is particularly of interest is zinc.
The most common zinc recycling process employs a rotary kiln. EAF dust is mixed with coal and charged to the kiln. Zinc oxide in the EAF dust is reduced to zinc metal, which boils or vaporizes off and then is oxidized to zinc oxide. The zinc oxide formed at this stage is of a low quality, but it can be upgraded by passing it through a second kiln and using other subsequent processing techniques. While the concept of recycling the constituents of EAF dust and other zinc bearing materials is encouraging in theory, it has proven too difficult and expensive.
Therefore, a need remains for economical processes for recovering, separating and recycling the constituents of EAF dust and other materials containing metal.
Briefly describing one aspect of the invention, a process for separating and recovering a desired metal as metal oxide from raw material containing metal oxides is provided. The process includes placing the raw material and a reductant in a container to form a reducing microclimate within the container. A housing is heated to maintain a temperature zone within the housing at a heating temperature sufficient to expose the raw material in the container to a reaction temperature. The reaction temperature is higher than the boiling point of the desired metal. The temperature zone within the housing has an oxidizing atmosphere. The process further includes conveying the container containing the raw material through the temperature zone in the housing to expose the raw material and the reductant to the reaction temperature wherein the metal oxide is reduced to a gaseous metal that exits the container. Once outside the container, the gaseous metal is exposed to the oxidizing atmosphere of the temperature zone wherein the desired metal is oxidized to metal oxide. The process also includes collecting the metal oxide.
In one specific embodiment, the housing is a tunnel kiln. In preferred embodiments, the raw material is EAF dust and the desired metal is zinc. Using the methods of this invention, zinc oxide collected from the housing has less than about 0.06% impurities and the iron-rich product has between less than about 1.0 % zinc and less than detectable levels of lead and cadmium. In some embodiments, the invention includes collecting volatilized halogens of lead and cadmium in a baghouse.
In some cases, the reaction temperature is at least about 900xc2x0 C. (1652xc2x0 F.), with one preferred temperature range for the reaction temperature being between about 1218xc2x0 C. (2225xc2x0 F.) and about 1649xc2x0 C. (3000xc2x0 F.). In one specific embodiment, the reductant contains carbon, such as coke or coal. In other embodiments, the reductant contains a metal, such as aluminum. In another aspect of this invention, the heating step includes maintaining the temperature zone at an oxygen level of at least about 2.0%. In other preferred embodiments, the heating step includes maintaining the partial pressure of carbon monoxide to carbon dioxide within the container at a ratio that is sufficient to achieve a reducing microclimate within the container.
In one specific embodiment, the process also includes maintaining a second temperature zone in the housing at a metal halide vaporization temperature, the metal halide temperature being lower than the heating temperature. A flow of air is applied through the housing in a direction that is opposite to a direction of travel of the container during the conveying step, whereby a metal halide in the raw material is volatilized to a volatilized metal halide when the container is conveyed through the second heating zone. The volatilized metal halide is then collected and separated from the metal oxide. The metal halide vaporization temperature is preferably between about 1600xc2x0 F. and about 2000xc2x0 F., with a most preferred temperature of about 1800xc2x0 F.
In embodiments in which the raw material does not contain halides, the methods can include mixing a halide with the raw material and the reductant, whereby the halide reacts with metal coumpounds in the raw material to form metal halides. In preferred embodiments, the halide is a chloride.
In certain preferred embodiments, the processes include mixing the raw material with the reductant before the placing step. The raw material and the reductant can be blended with a binding agent to form a blend and the blend formed into formed units, such as pellets or briquettes. In some embodiments the binding agent includes water. In some embodiments, the binding agent includes water present in the amount of about 3.0 wt % to about 20.0 wt %. In one particular embodiment, the raw material is present in the blend in an amount between about 65 wt % and about 80 wt %, the reductant is present in an amount between about 10 wt % and about 20 wt % and the binding agent is present in an amount between about 10 wt % and about 20 wt %. In preferred embodiments, the reductant is present in an amount sufficient to completely reduce the zinc oxide present in the electric arc furnace dust. In one specific embodiment, the raw material is present in the blend in an amount of about 84.5 wt %, the reductant is present in an amount of about 12.0 wt % and the binding agent is present in an amount of about 3.0 wt %.
In another specific embodiment, the placing step includes loading the formed units in the bed to a depth of between about 1.0 inches (2.5 cm) to about 14.0 inches (35.5 cm). In preferred embodiments, the placing step includes loading the formed units in the bed to a depth of between about 5.0 inches (12.7 cm) to about 9.0 inches (22.9 cm), with a most preferred depth being about 6.0 inches (15.2 cm). In some embodiments, the placing step includes loading the container with blend so that the container has a vent area/material volume ratio of at about 0.25 feet2 of vent area per cubic foot of blend within the bed.
In one aspect, the conveying step includes conveying the container on a kiln car and the collecting step includes collecting the metal oxide from the kiln car as the kiln car exits the housing. The conveying step, in one embodiment, includes conveying the container through the temperature zone in about one to about twenty hours. In one specific embodiment, the container is conveyed through the temperature zone in about ten to about fourteen hours.
Accordingly, it is one object of the invention to provide economical processes for recovering, separating and processing desired metals from metal bearing raw materials. Another object is to recover high quality zinc from EAF dust and other zinc bearing materials. These and other objects, advantages and features are accomplished according to the devices, assemblies and methods of the present invention. Other objects and further benefits of the present invention will become apparent to persons of ordinary skill in the art from the following written description and accompanying Figures.